Durations of the Photoperiod-sensitive and Photoperiod-insensitive Phases of Development to Flowering in Four Cultivars of Soyabean [Glycine max (L.) Merrill]

Four cultivars of soyabean [Glycine max (L.) Merill] of diverse origin were grown in pots in a plastic-house maintained at day/night temperatures of 30/20°C. Plants were transferred at various times after sowing from short (11·5 h d-1) to long (13·5 h d-1) days and vice versa. The times from sowing to first flowering for control plants grown continuously in short days varied from 38 to 53 d, whereas the flowering of plants grown continuously in long days was delayed by about 20 d in each cultivar. The duration of the initial photoperiod-insensitive phase (often called the juvenile phase) varied three-fold between cultivars, i.e. from 11 to 33 d. As expected, the duration of the photoperiod-sensitive phase was greater in long days, but there was comparatively little genetic variation in photoperiod-sensitivity as defined in terms of days delay in time to flowering per hour increase in photoperiod (9-11 d h-1). Similarly, there was little variation in the photoperiod-insensitive post-inductive phase; it ranged from 15 to 20 d. In consequence, the duration of the initial photoperiod-insensitive phase was a strong determinant of time to first flowering in these cultivars. The importance of this so-called juvenile trait is discussed in terms of preventing the premature flowering of USA-adapted cultivars when grown in short tropical daylengths and thus improving the adaptation of the crop to the lower latitudes.